I. Field of the Invention
The present invention relates to a rotating anode X-ray tube in which the anode may be rotated at a high rate of speed.
II. Background Information
Rotating anode X-ray tubes are known which comprise a vacuum housing and a rotating anode mounted within that housing. The rotating anode contains a target surface against which thermions emitted from a cathode are bombarded, with the energy of the thermions discharged as X-rays. The rotating anode is mounted within the vacuum housing to a rotor of a motor, with the rotor of the motor supported in the vacuum housing with a suitable bearing mechanism.
Anodes of such prior art rotating anode X-ray tubes are generally shaped in the form of an umbrella as illustrated in FIG. 1 by anode 10. Anode 10 of FIG. 1 may comprise molybdenum, either in essentially pure form or as an alloy. A ring-shaped target 12 made of tungsten, either in essentially pure form or as an alloy, is fittedly mounted in a surface of anode 10 adjacent the circumferential peripheral edge 14 of anode 10. Anode 10 is coupled to a support shaft 14 of a rotor of a motor (not shown) by having the distal end 16 of shaft 14 extend through a bore or opening 18 in anode 10 along the axis of rotation 20 of anode 10. Anode 10 is held in a fixed relation to shaft 14 by operation of nut 22 attached to that portion of distal end 16 of shaft 14 which extends beyond opening 18 of anode 10.
When employing an anode such as prior art anode 10 illustrated in FIG. 1, in order to get a clear and quality picture thermions need to be focused to strike target 12 in as small an area as possible and with as great a power input as possible. In order to achieve a large power input on a small focus area of target 12, the diameter of target 12 needs to be made as large as possible and/or anode 10 needs to be rotated as fast as possible. However, anode 10 is disposed inside a vacuum housing and, thus, the diameter of target 12 is limited by the internal dimensions of that vacuum housing. Accordingly, in order to obtain a clear and quality resultant X-ray picture, anode 10 needs to be rotated as fast as possible.
The prior art anode 10 of FIG. 1, however, has a limitation on the rate of rotation possible based upon the maximum centrifugal stress at the inside of anode 10 and the inherent manufacturing tolerances between the internal dimensions of bore 18 of anode 10 and the external dimensions of distal end 16 of shaft 14 which extends through that bore. The inherent manufacturing tolerances required to permit insertion of shaft 14 through bore 18 of anode 10 results in a certain amount of play between shaft 14 and anode 10 which, in turn, results in a backlash of anode 10 upon high speed rotation of anode 10, resulting in an imbalance of anode 10 and, therefore, a limitation on the ultimate rate of rotation which may be obtained by anode 10. Imbalance also results from any off-centering of bore 18 from the center of anode 10 and from any lack of symmetry in the roundness of bore 18.
Thus, the limitations on manufacturing tolerances existent in the positioning, roundness and size of bore 18 limits the ultimate rate of rotation which may obtained by anode 10 due to large centrifugal forces that affect the central portion of anode 10 along axis of rotation 20. Due to this limit on the rate of rotation which may achieved by anode 10, target 12 is subjected to more heat per unit area than would be the case at a higher rate of rotation and, thus, target 12 must be made thicker than would be the case if a higher rate of rotation of anode 10 were possible. Accordingly, the difference between the coefficient of expansion of the metal from which target 12 is constructed and the coefficient of expansion of the metal from which anode 10 is constructed, coupled with the necessarily thick dimensions of target 12, subjects target 12 to the likelihood of becoming separated from the surface of anode 10.